Most likely you have a Holley carb. You may have a 460 engine. If you have the carb with a big vacuum pot in front of the carb, then you have a RPM governor. The RPM governor will work against the 4 barrel.
The main mixture for the idle jets can be adjusted like any old style carb. You may have plastic caps over the idle screws but take them off. Then use a vacuum gauge on any intake fitting and proceed to read the vacuum level as you turn the mixture screws. You want the mixture to reach a maximum RPM, then evenly turn the screw in until the RPM drops slighty and then balance for smoothest idle.
After this step adjust curb idle and see if this helps when the 4 barrel kicks in. Holley has different springs for the vacuum actuated secondaries and they are color coded. If the carb is original it should be right, but if the carb is of unknown origin you can check the proper vacuum spring, main jet, power valve and so on in a good Holley book. The mechanical secondaries are also listed with clearances to hand adjust.
By adjusting the mixture screws you have set the engine to idle just below peak vacuum pressure. This means you should have more vacuum to work the secondaries.
For those like me with a good old fashioned Holley type carb, here's a real nice video on the basics. He's gonna be doing more in-depth videos in the future. This cat flat out knows carbs!
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there is no adjustment that makes the 4 barrels work, there is only air/fuel ratio.
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